Slideshows: FOUC when slideshow library has not loaded yet.

#### Steps to reproduce

1. Create a slideshow image gallery on a page
2. Load the page
3. See the error "This slideshow requires a Javascript" flash before your eyes, then disappear as the slideshow loads

#### What I expected

- No error message, just a slideshow

#### What happened instead

- The error message appears, then a slideshow

#### Additional Info

- You can see the action happen here in this GIF https://cloudup.com/cCFVMuWz1gg

- I've tested this with both Twenty Seventeen and Twenty Twelve to eliminate theme issues as well as Corporate. It happens on both a Simple and Atomic sites on WordPress.com.

- Reported by a user in #3280394-HC

No source file, test, or entry point is named. Reproduce the flash using the listed slideshow gallery steps, then trace how the slideshow library and its fallback message are loaded; done means the message never appears while the slideshow still initializes in the tested themes and site types.

Written by the indexing model from the issue text.